1. Field of the Invention
The present invention relates to the analysis of liquids which may contain interferents to the assay of the liquid under analysis.
2. Description of the Related Art
It is well known in the art to perform an analysis of a liquid by contacting that liquid with an analytical element containing either electrodes measuring the potential of or reagents capable of yielding a detectable product in proportion to the concentration of an analyte in the liquid. There are various useful methods of detecting the product, including colorimetric and potentiometric methods. The methods used can either entail the use of enzymatic reactions in liquid reagent solutions and barrel type electrodes in the ion-selective electrode method or they can be carried out in the more recent dry applications such as the multilayer element of U.S. Pat. No. 3,992,158 or the dry operative ion-selective electrode described in copending U.S. application Ser. No. 893,656 by Battaglia et al filed Apr. 5, 1978.
One particularly useful method involves an enzymatic assay wherein the analyte, upon contact with the analytical element, is oxidized in the presence of an enzyme contained therein to produce a peroxide in proportion to the concentration of the analyte in the liquid undergoing analysis. A detectable product is then produced by the reaction of the peroxide with an indicator composition in the presence of a substance having peroxidative activity. This detectable product is formed in direct proportion to the peroxide present and thus also in proportion to the concentration of the analyte. Elements and methods of this type are described in U.S. Pat. No. 3,992,158 and in U.S. Pat. No. 4,089,747 by B. J. Bruschi, both of which are incorporated herein by reference.
Methods of analyses employing reaction mechanisms other than the above-described peroxide mechanism to produce a detectable product are also known. For example, U.S. Pat. No. 3,711,252, describes a method for the quantitative analysis of uric acid in aqueous liquids wherein the aqueous liquid is contacted with a carrier element containing a ferric salt and either 2,4,6-tri(2-pyridyl)-1,3,5-triazine or 2,2':6',2"-terpyridyl, in a buffered acidic medium. A color change is produced which is directly proportional to the concentration of uric acid in the aqueous liquid.
Additional methods are described in U.S. Pat. No. 3,801,466.
Another particularly useful method includes the determination of potassium, sodium and other ions in a liquid sample using an ion-selective electrode comprising a metal/metal halide reference zone, an electrolyte layer and a membrane layer containing an ionophore for the ion to be analyzed. The potential is measured to determine concentration of the analyte. This method is described in the above-described U.S. patent application Ser. No. 893,656.
In all of the above-cited references to elements and methods for their use, it is also recognized that substances present in the liquid undergoing analysis other than the analyte may interfere with or bias the analytical reactions such that the detectable product is not formed in direct proportion to the analyte alone. This is particularly true for relatively low concentration analytes. For example, in analyzing for chloride ions using an ion-selective electrode, uric acid and bromide can interfere with the resulting potential; in potassium analysis using an ion-selective electrode, thiocyanate can interfere with the resulting potential; in the analysis of CO.sub.2, gentisate and aminosalicylate interfere; in the determination of bilirubin, hemoglobin and salicylate may interfere; and in the analysis of other materials such as cholesterol, bilirubin is a known interferent.
The above interference problems are significant because these interferents are commonly found in liquids which are desirably analyzed such as blood serum and urine. Many interferents can also be present in body fluids from the ingestion of certain medications, for example, aspirin. The interferents can result in false detection of the analyte.
Methods are available and known to avoid interferences of this type. For example, U.S. Pat. No. 3,711,252, suggests prevention of gentisic acid interference by incorporation of persulfate in the analytical element. U.S. Pat. No. 3,801,466 suggests a multi-step method of avoidance involving preparation of comparative test samples in one of which the analyte is totally eliminated by a preanalysis reaction. The two samples are then analyzed for analyte, and the difference in results between the two indicates the concentration of interferents that may be present.
U.S. patent application Ser. No. 956,527 to Battaglia, Secord and Kim, filed Oct. 31, 1978, describes a method of removing uric acid and bromide ions from a potentiometric analysis of chloride ions by using an overcoat layer of a polymeric material, such as cellulose esters. The interfering bromide ions are removed by slowing down the diffusion of the interferent through the overcoat.
U.S. patent application Ser. No. 907,640 by Wu, filed May 19, 1978, describes the preparation of enzymes to degrade bilirubin and remove it as an interferent.
U.S. patent application Ser. No. 848,255 of Schubert, filed Nov. 3, 1977, describes reduction of gentisic acid interference by dissolving an indicator composition in an organic solvent.
While the above methods to alleviate interferences are useful, they are either inconvenient to use as they involve multiple steps or they are applicable to only one method of analysis.
U.S. Pat. No. 3,617,546 describes a water purification method wherein a sample of waste water is mixed in a single membrane comprising an emulsion to remove impurities. A multizone or multilayer element is not disclosed and no method of analysis is involved.
Accordingly, it would be desirable to provide an analytical element comprising multiple zones which would be effective in eliminating biases due to interferences from a variety of interferents and which would be useful for a wide variety of analytical methods.